Video proxy server

ABSTRACT

A video proxy server capable of streaming video data after a seek position in quick response to a request from a terminal while caching the video data without any loss. When the video proxy server receives a seek playback request while executing a video reading and caching operation of receiving video data from a content server, caching the video data, and streaming the cached video data to a terminal, the video proxy server starts receiving video data after the seek position from the content server, caching the video data after the seek position, and streaming the video data after the seek position to the terminal while continuing to receive and cache the video data so as to prevent any loss of the video data as the video data after the seek position is concurrently received, cached and streamed to the terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to video proxy servers and, morespecifically, to a video proxy server provided between a content serverand a terminal, for receiving video data from the content server,caching the video data, and then streaming the video data to theterminal for playback.

2. Description of the Background Art

In recent years, significant improvements in computer performance haveboosted video distribution services through the Internet for businessand amusement purposes. Such video distribution services often use atechnology called streaming. In video distribution with streaming, videodata is stored in a server. In response to a playback request from aterminal, the server distributes requested video data to the terminal.In this case, it takes time for the terminal to receive the requestedvideo data after transmitting the request. Such a response time lagbetween transmission of the request and the start of reproducing therequested video data should be reduced.

Conventionally, to reduce the response time, a scheme of providing adevice called a proxy server between a server and terminals has beensuggested. The proxy server is placed in the vicinity of the terminalsin order to quickly respond to a request therefrom, and caches (stores)video data coming from the server for streaming to the terminals. Then,when any terminal requests the same video data as that which waspreviously cached in the proxy server, the cached video data is streamedto the requesting terminal, thereby achieving a reduction in responsetime.

FIG. 14 is a block diagram illustrating an example configuration of asystem using a conventional video proxy server. This system isconstructed on a network by original video data 10 stored in a server orothers on the network, a video proxy server 12 for caching the videodata 10, and a terminal 11 connected to the video proxy server 12 forreceiving the video data 10. In FIG. 14, only one terminal 11 isillustrated, but a plurality of terminals may be provided. The operationof the conventional video proxy server 12 is described in further detailbelow with reference to FIG. 14.

In FIG. 14, the video proxy server 12 includes a cache 14 for storingthe video data 10 and a cache controller 13 for controlling the cache14. The cache controller 13 causes the cache 14 to cache the originalvideo data 10 based on a playback start request coming from the terminal11.

In this conventional art, the original video data 10 requested by theterminal 11 is streaming thereto while being cached by the video proxyserver 12. Then, when the terminal 11 again requests the same video dataas the original video data 10, the video proxy server 12 streams thecached video data to the terminal 11. This enables a reduction in theamount of time that is taken to respond to a playback request, ascompared with a case where the original video data 10 is streaming fromthe server. Such a configuration is disclosed, for example, in JapanesePatent Laid-Open Publication No. 9-81491 (1997-81491).

However, the conventional video proxy server 12 has a drawback. Considera case where the video proxy server 12 receives a seek playback request(request for changing a data position for playback) for seeking aposition of data not yet cached while reading the original video data 10from the server upon request from the terminal 11, caching the videodata, and streaming the video data to the terminal 11. In this case, forstreaming to the terminal 11, the video proxy server 12 has to waituntil video data after a requested position (hereinafter, seek position)is cached. This causes an increase in response time in proportion to adistance (time difference) between the seek position requested by theterminal 11 and a current playback position.

For eliminating such an increase in response time, the position to beread in the original video data 10 should be quickly changed to therequested seek position. Then, the video data after the seek position isstreaming to the terminal 11 while being cached. In this case, however,the video data cached in the video proxy server 12 may miss some data(i.e., become discontinuous in time), and therefore is not reusable and,as a result, becomes wasted.

Moreover, there is another drawback. In the conventional video proxyserver 12, a cache speed of reading the original video data 10 from theserver for caching is predetermined. Therefore, if the plurality ofterminals 11 set different playback speeds or change their playbackspeeds during playback, those playback speeds cannot exceed thepredetermined cache speed.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a videoproxy server which is capable of streaming video data after a seekposition in quick response to a request from a terminal while cachingthe video data without any loss.

Another object of the present invention is to provide a video proxyserver which is capable of streaming video data to each terminal at aspeed corresponding to each desired playback speed.

The present invention has the following features to attain the objectsmentioned above.

A first aspect of the present invention is directed to a video proxyserver, which is provided between a content server storing video dataand a terminal capable of video playback, for receiving the video datafrom the content server, caching the video data, and streaming the videodata to the terminal for playback. The video proxy server includes: arequest receiving section for receiving a request from the terminal; avideo receiving section for receiving the video data from the contentserver based on the request from the terminal; a video cache section forcaching the video data received by the video receiving section; and avideo streaming section for streaming the video data cached by the videocache section to the terminal.

When the request receiving section receives a playback request, thevideo receiving section starts a video receiving operation of receivingthe video data from the content server. When the request receivingsection receives a seek playback request while the video receivingsection is executing the video receiving operation, the video receivingsection starts receiving video data after a seek position from thecontent server while continuing the video receiving operation.

The video cache section includes a cache area for caching the video datareceived by the video receiving section from the content server, and atemporary cache area for temporarily caching the video data after theseek position received by the video receiving section from the contentserver.

In the first aspect (or the following twelfth and thirteenth aspects),when a seek playback request is issued from the terminal to which thevideo data received from the content server and cached in the cache areais streaming, an operation of receiving the video data after the seekposition from the content server is newly started. At this time, theoperation of receiving the video data from the content server andcaching the video data in the cache area is not stopped but continued.

That is, when the terminal does a seek during video playback, the videoproxy server concurrently executes the operation of receiving andcaching the video data and the operation of receiving the video dataafter the seek position and temporarily caching the video data after theseek position. Thus, it is possible to cache the video data without anyloss even with seek playback at the terminal.

The cache area and the temporary cache area are implemented by one pieceof RAM in the preferred embodiments. Alternatively, these areas may beimplemented by two or more pieces of RAM, or may be implemented by astorage medium other than RAM which is capable of quick reading andwriting.

In the second aspect, in accordance with the first aspect, when therequest receiving section receives the playback request, the videostreaming section starts reading the video data from the cache area andstreaming the video data to the terminal. Furthermore, when the requestreceiving section receives the seek playback request while the videoreceiving section is receiving the video data from the content server,the video streaming section stops reading the video data from the cachearea and streaming the read video data to the terminal, and startsreading the video data cached in the temporary cache area and streamingthe read video data to the terminal.

In the second aspect, when a seek playback request is issued from theterminal to which the video data received from the content server andcached in the cache area is being streamed, an operation of receivingvideo data after the seek position from the content server, caching thevideo data after the seek position in the temporary cache area, andstreaming the video data after the seek position to the terminal isnewly started. At this time, the operation of receiving the video datafrom the content server and caching the video data in the cache area isnot stopped but continued. Only the operation of streaming the videocached in the cache area is stopped.

That is, when the terminal does a seek during video playback, the videoproxy server concurrently executes the operation of receiving andcaching the video data, and the operation of receiving the video dataafter the seek position and temporarily caching the video data after theseek position. Thus, it is possible to stream the video data after theseek position in quick response to the request from the terminal, whilecaching the video data without any loss even with seek playback at theterminal.

According to a third aspect, in accordance with the first aspect, whenthe request receiving section receives a stop request while the videoreceiving section is receiving the video data after the seek positionfrom the content server in response to the seek playback request, thevideo cache section clears the temporary cache area.

In the third aspect, it is possible to quickly respond to the next seekplayback request.

According to a fourth aspect, in accordance with the first aspect, whenthe request receiving section receives a next seek playback requestwhile the video receiving section is receiving the video data after theseek position from the content server in response to the seek playbackrequest, the video cache section clears the temporary cache area, andthen starts caching the video data after a next seek position in thetemporary cache area.

In the fourth aspect, it is possible to quickly respond to incoming seekplayback requests by using the limited temporary cache area.

According to a fifth aspect, in accordance with the first aspect, thevideo receiving section receives the video data or the video data afterthe seek position based on the request from the terminal only when therequested video data or the requested video data after the seek positionhas not been cached by the video cache section.

In the fifth aspect, when the requested video data or the requestedvideo data after the seek position has been cached in the video cachesection, the operation of receiving the video data from the contentserver is not required. Therefore, it is possible to reduce the amountof time that is taken to start streaming after the request is received,and also save a band of the network.

A sixth aspect of the present invention is directed to a video proxyserver, which is provided between a content server storing video dataand a plurality of terminals capable of video playback at playbackspeeds that are different from each other, for receiving the video datafrom the content server, caching the video data, and streaming the videodata to the terminals for playback. The video proxy server includes: aspeed managing section for managing the playback speeds of theterminals; a video receiving section for receiving the video data fromthe content server; a video cache section for caching the video datareceived by the video receiving section; and a video streaming sectionfor streaming the video data cached by the video cache section to theterminals at speeds corresponding to the respective playback speeds ofthe terminals.

The video receiving section receives the video data from the contentserver at a speed Vmax corresponding to a speed that is the highest ofthe playback speeds of the terminals managed by the speed managingsection.

In the sixth aspect (or the following fourteenth and fifteenth aspects),the playback speeds of the terminals are managed, and video data isreceived from the content server at a speed corresponding to the onethat is the highest of the playback speed of the terminals, cached, andstreamed to the terminals. Thus, it is possible to stream video data ata speed corresponding to the playback speed of each terminal.

According to a seventh aspect, in accordance with the sixth aspect, thevideo proxy server further includes a request receiving section forreceiving a request from each of the terminals. When the requestreceiving section receives a speed-change request for changing the speedVmax at which the video data is being received from the content serverto a speed V, the video receiving section compares the speed V with thespeed Vmax and, when the speed V>the speed Vmax, moves to an operationof receiving the video data from the content server at the speed V.

According to an eighth aspect, in accordance with the seventh aspect,when the speed V<the speed Vmax, the video receiving section continuesreceiving the video data from the content server at the speed Vmax.

In the above seventh and eighth aspects, when any of the terminalschanges the playback speed to the speed V, the speed V is compared withthe highest speed Vmax before the change. If the speed V>the speed Vmax,the speed of video reception from the content server is changed fromVmax to V. On the other hand, if the speed V<the speed Vmax, the speedis maintained at Vmax. Thus, it is possible to stream video data at aspeed corresponding to the playback speed of each terminal.

According to a ninth aspect, in accordance with the sixth aspect, thevideo proxy server further includes a request receiving section forreceiving a request from each of the terminals. When the requestreceiving section receives a stop request while the video receivingsection is receiving the video data from the content server at the speedVmax, the video receiving section moves to an operation of receiving thevideo data from the content server at a speed V′max corresponding to aspeed that is the highest of the playback speeds, managed by the speedmanaging section, of the terminals except for the terminal that made thestop request.

In the ninth aspect, when any of the terminals is stopped, the speed ofvideo reception from the content server becomes the one that correspondsto the highest one of the playback speeds of the remaining terminals.That is, when the highest terminal is stopped, the speed of videoreception from the content server is changed to the speed V′maxcorresponding to the playback speed of the second highest terminal. Whenthe terminal other than the highest one is stopped, the receiving speedis maintained at the speed Vmax. Thus, it is possible to stream videodata at a speed corresponding to the playback speed of each terminalirrespectively of which terminal is stopped.

According to a tenth aspect, in accordance with the sixth aspect, thevideo proxy server further includes a request receiving section forreceiving a request from each of the terminals. When the requestreceiving section receives a stop request while the video receivingsection is receiving the video data from the content server at the speedVmax, the video receiving section compares a speed V′max correspondingto a speed that is the highest of the playback speeds, managed by thespeed managing section, of the terminals except for the terminal thatmade the stop request with a predetermined speed Vdef, and when thespeed V′max>the speed Vdef, the video receiving section moves to anoperation of receiving the video data from the content server at thespeed V′max

According to an eleventh aspect, in accordance with the tenth aspect,when the speed V′max<the speed Vdef, the video receiving section movesto an operation of receiving the video data from the content server atthe speed Vdef.

In the tenth and eleventh aspects, even if the playback speed of any ofthe terminals becomes extremely low, the speed of video reception fromthe content server is maintained at the speed Vdef or higher forcaching. Therefore, it is possible to respond to a sudden seek at any ofthe terminals.

A twelfth aspect of the present invention is directed to a videostreaming method which is carried out between a content server storingvideo data and a terminal capable of video playback, for receiving thevideo data from the content server, caching the video data, andstreaming the video data to the terminal for playback. The methodincludes the steps of: receiving a request from the terminal; receivingthe video data from the content server based on the request from theterminal; caching the video data received in the video receiving step;and streaming the video data cached in the video caching step to theterminal.

In the video receiving step, when a playback request is received in therequest receiving step, a video receiving operation of receiving thevideo data from the content server is started, when a seek playbackrequest is received in the request receiving step while the videoreceiving operation is being executed, an operation of receiving videodata after a seek position from the content server is started while thevideo receiving operation is being continued.

In the video caching step, when the playback request is received in therequest receiving step, a video caching operation of caching the videodata received in video receiving step from the content server isstarted, and when the playback request is received in the requestreceiving step while the operation of receiving the video data from thecontent server is being executed, an operation of temporarily cachingthe video data after the seek position received in video receiving stepfrom the content server while the video caching operation is beingexecuted is continued.

A thirteenth aspect of the present invention is directed to acomputer-readable medium having a program stored thereon that describesa video streaming method which is carried out between a content serverstoring video data and a terminal capable of video playback, forreceiving the video data from the content server, caching the videodata, and streaming the video data to the terminal for playback. Themethod includes the steps of: receiving a request from the terminal;receiving the video data from the content server based on the requestfrom the terminal; caching the video data received in the videoreceiving step; and streaming the video data cached in the video cachingstep to the terminal.

In the video receiving step, when a playback request is received in therequest receiving step, a video receiving operation of receiving thevideo data from the content server is started, when a seek playbackrequest is received in the request receiving step while the videoreceiving operation is being executed, an operation of receiving videodata after a seek position from the content server is started while thevideo receiving operation is being continued.

In the video caching step, when the playback request is received in therequest receiving step, a video caching operation of caching the videodata received in video receiving step from the content server isstarted, and when the playback request is received in the requestreceiving step while the operation of receiving the video data from thecontent server is being executed, an operation of temporarily cachingthe video data after the seek position received in video receiving stepfrom the content server while the video caching operation is beingexecuted is continued.

A fourteenth aspect of the present invention is directed to a videostreaming method which is carried out between a content server storingvideo data and a plurality terminals capable of video playback atplayback speeds that are different from each other, for receiving thevideo data from the content server, caching the video data, andstreaming the video data to the terminals for playback. The methodincludes the steps of: managing the playback speeds of the terminals;receiving the video data from the content server; caching the video datareceived in the video receiving step; and streaming the video datacached by the video caching step to the terminal at speeds correspondingto the respective playback speeds of the terminals.

In the video receiving step, the video data is received from the contentserver at a speed Vmax corresponding to a speed that is the highest ofthe playback speeds of the terminals managed in the speed managing step.

A fifteenth aspect of the present invention is directed to acomputer-readable medium having a program stored thereon that describesa video streaming method which is carried out between a content serverstoring video data and a plurality terminals capable of video playbackat playback speeds differed from each other, for receiving the videodata from the content server, caching the video data, and streaming thevideo data to the terminals for playback. The method includes the stepsof: managing the playback speeds of the terminals; receiving the videodata from the content server; caching the video data received in thevideo receiving step; and streaming the video data cached by the videocaching step to the terminals at speeds corresponding to the respectiveplayback speeds of the terminals.

In the video receiving step, the video data is received from the contentserver at a speed Vmax corresponding to a speed that is the highest ofthe playback speeds of the terminals managed in the speed managing step.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of a videostreaming system using a video proxy server according to a firstembodiment of the present invention;

FIG. 2 is a block diagram illustrating an example hardware structure ofthe video proxy server according to the first embodiment of the presentinvention;

FIG. 3 is a flowchart showing the operation which is carried out by thevideo proxy server according to the first embodiment of the presentinvention for seek playback;

FIG. 4 is a flowchart showing step S14 of FIG. 3 (or step S24 of FIG.10) in detail;

FIG. 5 is a flowchart showing step S16 of FIG. 3 in detail;

FIG. 6 is a conceptual illustration visually showing the operation ofstep S16 of FIG. 3;

FIG. 7 is a flowchart showing step S18 of FIG. 3 in detail;

FIG. 8 is a conceptual illustration visually showing the operation ofstep S18 of FIG. 3;

FIG. 9 is a block diagram showing the configuration of a video streamingsystem using a video proxy server according to a second embodiment ofthe present invention;

FIG. 10 is a flowchart showing the operation which is carried out by thevideo proxy server according to the second embodiment of the presentinvention for speed change;

FIG. 11 is a flowchart showing step S26 of FIG. 10 in detail;

FIG. 12 is a flowchart showing a process denoted as “A” in FIG. 11 ofstep S26 of FIG. 10 in detail;

FIG. 13 is a flowchart showing a process denoted as “B” in FIG. 11 ofstep S26 of FIG. 10 in detail; and

FIG. 14 is a block diagram showing an example configuration of astreaming system using a conventional video proxy server.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, described below are embodiments of thepresent invention.

First Embodiment

FIG. 1 is a block diagram illustrating the configuration of a videostreaming system using a video proxy server according to a firstembodiment of the present invention. In FIG. 1, the video streamingsystem includes a content server 20, a terminal 21, and a video proxyserver 22 according to the present embodiment. The terminal 21 isconnected to the video proxy server 22 via a network 31. The video proxyserver 22 is connected to the content server 20 via a network 32.

The content server 20 stores video data. The terminal 21 reproducesvideo data streaming thereto. The video proxy server 22 is providedbetween the content server 20 and the terminal 21, receives video datafrom the content server 20, caches the received video data, and streamsthe video data to the terminal 21.

The video proxy server 22 includes a request receiving section 23, avideo receiving section 24, a video cache section 25, and a videostreaming section 26.

The request receiving section 23 receives a request from the terminal21. The video receiving section 24 receives video data from the contentserver 20 based on the request from the terminal 21. The video cachesection 25 caches the video data received by the video receiving section24. The video streaming section 26 streams the video data cached by thevideo cache section 25 to the terminal 21.

Here, the video data stored in the content server 20 is digital videodata, such as movies, live concerts and sports. The content server 20has a function of sending out, in response to a request from theterminal 21, video data as a stream. The video data sent out from thecontent server 20 is streaming through the networks 31 and 32 to theterminal 21, which reproduces the streaming video data on a real-timebasis.

The terminal 21 can carry out two types of playback. One is normalplayback of successively reproducing video data from its head position.The other is seek playback of starting playback from a position (seekposition) other than the head position. For normal playback, theterminal 21 issues a playback request, and the content server 20 sendsout the requested video data from its head position. For seek playback,the terminal 21 issues a seek playback, and the content server 20 sendsout the requested video data from the seek position. To make playbackstop, the terminal 21 issues a stop request, and the content server 20stops sending out the video data.

The video cache section 25 includes a cache area 25 a for caching thevideo data received by the video receiving section 24 from the contentserver 20, and a temporary cache area 25 b for temporarily caching thevideo data after the seek position received by the video receivingsection 24 from the content server 20.

The video receiving section 24 receives the video data or the video dataafter the seek position based on the request from the terminal 21, fromthe video cache section 25 if cached, or from the content server 20 ifnot cached. This enables reduction in the amount of time that is takento start streaming after the request is received, and also can save loadon the bandwith of the network 32.

In other words, when receiving a playback request, the video receivingsection 24 determines whether the requested video data has been cachedin the video cache section 25. If the requested video data has not yetbeen cached in the video cache section 25, the video receiving section24 starts a video receiving operation of receiving the video data fromthe content server 20. During the video receiving operation, when therequest receiving section 23 receives a seek playback request, the videoreceiving section 24 starts an operation of temporarily receiving thevideo data from the seek position from the content server 20 whilecontinuing the video receiving operation.

When the request receiving section 23 receives a playback request, thevideo streaming section 26 starts reading the requested video data fromthe cache area 25 a for streaming to the terminal 21. Then, when therequest receiving section 23 receives a seek playback request while thevideo receiving section 24 is carrying out the video receiving operationof receiving the video data from the content server 20, the videostreaming section 26 stops reading the requested video data from thecache area 25 a for streaming to the terminal 21, and starts reading therequested video data from the temporary cache area 25 b for streaming tothe terminal 21.

In other words, when a seek playback request is issued from the terminal21 while the video data is being received from the content server 20,being cached in the cache area 25 a, and being streaming to the terminal21, the video proxy server 22 starts receiving the video data after theseek position from the content server 20, caches the video data afterthe seek position in the temporary cache area 25 b, and streams thevideo data after the seek position to the terminal 21. At this time, thevideo proxy server 22 continues receiving the video data from thecontent server 20 and caches the video data in the cache area 25 a, butstops streaming the video data cached in the cache area 25 a to theterminal 21.

That is, when the terminal 21 does a seek during video playback, thevideo proxy server 22 concurrently carries out the operation ofreceiving the video data and caching the video data, and the operationof receiving the video data after the seek position, temporarily cachingthe video data after the seek position, and streaming the video dataafter the seek position to the terminal 21. This enables a quickresponse to the seek playback request from the terminal 21, and alsoenables continuous playback of the video data being cached.

Furthermore, when the request receiving section 23 receives a nextplayback request while the video receiving section 24 is receiving thevideo data after the seek position from the content server 20 inresponse to the seek playback request, the video cache section 25 clearsthe temporary cache area 25 b, and then starts an operation of cachingthe video data after the subsequent seek position in the temporary cachearea 25 b. This enables a quick response to incoming seek playbackrequests by using the limited temporary cache area 25 b.

FIG. 2 is a block diagram illustrating an example hardware structure ofthe video proxy server 22 according to the present invention. In FIG. 2,the video proxy server 22 includes a CPU 33, ROM 34, RAM 35, and anetwork connection board 36. The network connection board 36 connectsthe video proxy server 22 and the networks 31 and 32 together. The ROM34 stores a predetermined program. When the CPU 33 executes the programstored in the ROM 34 while using the RAM 35 as a working area, theabove-described operations are carried out by the components (23 to 26)of the video streaming system of the first embodiment. Although theprogram is previously stored in the ROM 34 in the present embodiment,the program may be provided as being stored in a portable recordingmedium such as CD-ROM, or may be supplied via a network.

Described next below is the operation which is carried out by theabove-constructed video proxy server 22 for seek playback. FIG. 3 is aflowchart showing the operation which is carried out by the video proxyserver 22 according to the present embodiment for seek playback. Asillustrated in FIG. 3, in the video proxy server 22, the requestreceiving section 23 first determines whether any request has come fromthe terminal 21 (step S11). If any request has come, the requestreceiving section 23 receives the request (step S12). Then, the requestreceiving section 23 determines whether the request is a playbackrequest (step S13) If the request is not a playback request, theprocedure goes to step S15.

If it is determined in step S13 that the request is a playback request,the operation of receiving the video data from the content server 20,caching the video data, and streaming the video data to the terminal 21is started (step S14).

In the next step S15, it is determined whether the request received instep S12 is a seek playback request. If the request is not a seekplayback request, step S17 is executed.

If it is determined in step S15 that the request is a seek playbackrequest, the operation of receiving the video data after the seekposition from the content server 20, temporarily caching the video dataafter the seek position, and streaming the video data after the seekposition to the terminal 21 is started (step S16).

In the next step S17, it is determined whether the request received instep S12 is a stop request or a next seek playback request. If therequest is neither a stop request nor a next seek playback request, theprocedure returns to step S11, where the request receiving section 23waits until another request comes from the terminal 21.

If it is determined in step S17 that the request is a stop request, theoperation is stopped. If it is determined in step S17 that the requestis a next seek playback request, the operation of receiving, from thecontent server 20, the video data from the next seek position,temporarily caching the video data from the next seek position, andstreaming the video data from the next seek position to the terminal 21is started (step S18). Thereafter, the procedure returns to step S11,where the request receiving section 23 waits until another request comesfrom the terminal 21.

The operation which is carried out by the video proxy server 22 for seekplayback has now been described. Described next are details about theaforementioned steps S14, S16, and S18.

[Step S14]

FIG. 4 is a flowchart showing in detail the operation in step S14 ofFIG. 3 which is carried out by the video proxy server 22 for receivingvideo data from the content server 20, caching the video data, andstreaming the video data to the terminal 21. In FIG. 4, the videoreceiving section 24 first determines whether the requested video datahas already been cached in the video cache section 25 (step S101). Ifthe requested video data has been cached in the video cache section 25,the procedure goes to step S104.

If it is determined in step S101 that the requested video data has notyet been cached in the video cache section 25, the video receivingsection 24 starts receiving the video data from the content server 20(step S102). Simultaneously, the video cache section 25 starts cachingthe video data received by the video receiving section 24 in the cachearea 25 a (step S103). On the other hand, the video streaming section 26starts reading the video data cached in the cache area 25 a, and streamsthe read video data to the terminal 21 (step S104). The procedure thenreturns to the flow of FIG. 3.

[Step S16]

FIG. 5 is a flowchart showing in detail the operation in step S16 ofFIG. 3 which is carried out by the video proxy server 22 for receivingthe video data after the seek position from the content server 20,temporarily caching the video data after the seek position, andstreaming the video data after the seek position to the terminal 21.FIG. 6 is a conceptual illustration visually illustrating the operationof step S16 of FIG. 3. With reference to FIG. 5, when a seek playbackrequest is outputted from the terminal 21, the request receiving section23 receives the request (step S201).

Upon receiving the seek playback request, the request receiving section23 instructs the video receiving section 24 to retrieve video data afterthe seek position from the content server 20 (step S202). In response,the video receiving section 24 checks whether the video data after theseek position (the seek position is specified with time, such as “60minutes later”) has been cached in the video cache section 25 (stepS203).

If it is determined in step S203 that the video data after the seekposition has been cached in the video cache section 25, the videoreceiving section 24 instructs the video streaming section 26 to startreading the cached video data after the seek position from the videocache section 25 and streaming the read video data after the seekposition to the terminal 21 (step S207) The procedure then returns tothe flow of FIG. 3.

If it is determined in step S203 that the video data after the seekposition has not yet been cached in the video cache section 25, thevideo receiving section 24 carries out the following operation. That is,in FIG. 6, video data 42 successive in time (“1”, “2”, . . . ) isstreaming from the content server 20. Here, the video receiving section24 is receiving the video data 42 while instructing the video cachingsection 25 to cache the video data 42 in the cache area 25 a. That is,in the cache area 25 a of the video cache section 25, the video data isbeing cached successively from the head position, such as “1”, “2”, . .. (refer to a solid-line arrow 43 in FIG. 6).

In this state, if it is determined in step S203 that the video dataafter the seek position has not yet been cached, the content server 20is instructed to newly stream the requested video data after the seekposition “M”, “M+1”, . . . (step S204). In this way, the video receivingsection 24 starts receiving the video data after the seek position whilecontinuing to receive the video data 42 from the head position.

Of the video data streaming from the content server 20, the video cachesection 25 stores only the newly requested video data after the seekposition “M”, “M+1”, . . . in the temporary cache area 25 b as temporaryvideo data (step S205). That is, the video cache section 25 caches thevideo data after the seek position “M”, “M+1”, . . . in the temporarycache area 25 b while continuing to cache the video data from the headposition, such as “1”, “2”, in the cache area 25 a.

A flow of the video data streaming in steps S204 and S205 isconceptually denoted with a dotted arrow 44 in FIG. 6. A portion of thevideo data 42 after the seek position (in FIG. 6, M and thereafter) isreceived from the content server 20, and stored, as temporary videodata, in the area (temporary cache area 25 b), which is not the area forstoring the video data from the head position (cache area 25 a).

Then, the request receiving section 23 instructs the video streamingsection 26 to read and stream the temporary video data from the videocache section 25 (step S206). The procedure then returns to the flow ofFIG. 3.

[Step S18]

FIG. 7 is a flowchart showing in detail the operation in step S18 ofFIG. 3 which is carried out by the video proxy server 22 for receivingthe video data after the next seek position from the content server 20,temporarily caching the video data after the next seek position, andstreaming the video data after the next seek position to the terminal21. FIG. 8 is a conceptual illustration visually showing the operationof step S18 of FIG. 3. With reference to FIG. 7, the video streamingsection 26 is now streaming the video data after the seek position asthe temporary video data in response to the seek playback request fromthe terminal 21 (step S301). In this state, the request receivingsection 23 receives, from the terminal 21, a request for stopping videostreaming (stop request) or a seek playback request for seeking anotherposition in the video data being streamed (next seek playback request)(step S302).

Based on the stop request or the next seek playback request, the requestreceiving section 23 instructs the video streaming section 26 to stopstreaming of the temporary video data in response to the previous seekplayback request (step S303). The request receiving section 23 theninstructs the video receiving section 24 to stop receiving the temporaryvideo data. In response, the video receiving section 24 requests thecontent server 20 to stop sending out the temporary video data (stepS304). The request receiving section 23 also instructs the video cachesection 25 to delete, from the temporary cache area 25 b, the temporaryvideo data cached based on the previous seek playback request (that is,to clear the temporary cache area 25 b) (step S305).

With reference to FIG. 8, the operation of the above steps S304 and S305is described. In FIG. 8, video data 62 successive in time (“1”, “2”, . .. ) is being sent out from the content server 20. In the cache area 25a, the video data 62 is being cached successively from the headposition, such as “1”, “2”, . . . (refer to a solid-line arrow 63 inFIG. 8).

At the same time, in response to the previous seek playback request,video data after the seek position “M”, “M+1”, . . . is being sent outfrom the content server 20. In the temporary cache area 25 b of thevideo cache section 25, the video data after the seek position “M”,“M+1”, . . . is being cached as the temporary video data (refer to adotted arrow 64 in the drawing).

In this state, when a next seek playback request is issued from theterminal 21, the operation of receiving the temporary video data “M”,“M+1”, . . . being executed by the video receiving section 24 inresponse to the previous seek playback request (that is, the operationof sending out the temporary video data “M”, “M+1”, . . . being executedby the content server 20; refer to the dotted arrow 64 in FIG. 8) isstopped. Then, the temporary video data “M”, “M+1”, . . . being storedin the temporary cache area 25 b of the video cache section 25 isdeleted (that is, the temporary cache area 25 b is cleared).

Then, the operation of receiving temporary video data “N”, “N+1”, . . .in response to the next seek playback request is newly started. Thetemporary video data “N”, “N+1”, . . . is being newly stored in thecleared temporary cache area 25 b (refer to a dotted arrow 65 in thedrawing).

Referring back to FIG. 7, described below is the procedure that issubsequent to step S305. If the request received in step S302 by therequest receiving section 23 is a stop request, the procedure returns tothe flow of FIG. 3.

Furthermore, the request receiving section 23 determines whether thevideo data after the next seek position has been cached in the videocache section 25 (step S307). If the video data after the next seekposition has been cached in the video cache section 25, the requestreceiving section 23 instructs the video streaming section 26 to readthe video data after the next seek position from the video data alreadycached in the video cache section 25 and start streaming the cachedvideo data after the next seek position to the terminal 21 (step S311)The procedure then returns to the flow of FIG. 3.

If the request received in step S302 by the request receiving section 23is not a stop request but a next seek playback request and it isdetermined in step S307 that the video data after the next seek positionhas not yet been cached in the video cache section 25, the videoreceiving section 24 requests the content server 20 to newly stream thevideo data after the next seek position (step S308). The video cachesection 25 then stores, of the video data sent out from the contentserver 20, only the data sent out in response to the request made instep S308 in the temporary cache area 25 b as temporary video data (stepS309).

A flow of the video data streaming in the above steps S308 and S309 isconceptually denoted with a dotted arrow 65 in FIG. 8. A portion of thevideo data 62 after the next seek position (in FIG. 8, N and thereafter)is received from the content server 20, and stored, as temporary videodata, in the area (temporary cache area 25 b), which is not the area forstoring the video data 62 from the head position (cache area 25 a).

Finally, the request receiving section 23 instructs the video streamingsection 26 to read the temporary video data from the temporary cachearea 25 b of the video cache section 25 and stream the temporary videodata to the terminal 21 (step S310) The procedure then returns to theflow of FIG. 3.

As such, according to the present embodiment, when the terminal 21 doesa seek during video playback, the operation of receiving and caching thevideo data, and the operation of receiving video data after the seekposition, temporarily caching the video data after the seek position,and streaming the video data after the seek position to the terminal 21are concurrently carried out. Therefore, it is possible to quicklyrespond to the request from the terminal for streaming the video dataafter the seek position, and it is also possible to cache the video datawithout any loss.

Second Embodiment

FIG. 9 is a block diagram illustrating the configuration of a videostreaming system using a video proxy server according to a secondembodiment of the present invention. In FIG. 9, the video streamingsystem includes a content server 70, terminals 71 ₁ to 71 _(k) (where kis an arbitrary integer not less than 2), and a video proxy server 72according to the present embodiment. The terminals 71 ₁ to 71 _(k) areconnected to the video proxy server 72 via a network 81. The video proxyserver 72 is connected to the content server 70 via a network 82. Notethat, in the following description, the terminals 71 ₁ to 71 _(k) arecollectively denoted as “terminals 71” if they do not have to bedistinguished from each other.

The content server 70 stores video data. Each terminal 71 reproducesvideo data streaming thereto. The video proxy server 72 is placedbetween the content server 70 and the terminals 71, receives video datafrom the content server 70, caches the video data, and streams the videodata to the terminals 71.

The video proxy server 72 includes a request receiving section 73, avideo receiving section 74, a video cache section 75, a video streamingsection 76, and a speed managing section 77.

The request receiving section 73 receives a request from any of theterminals 71. The video receiving section 74 receives video data fromthe content server 70. The video cache section 75 caches the video datareceived by the video receiving section 74. The video streaming section76 streams the video data cached by the video cache section 75 to theterminals 71.

Here, the video data stored in the content server 70 is digital videodata, as in the first embodiment. The content server 70 has a functionof sending out video data as a stream in response to a request from anyof the terminals 71. The video data sent out from the content server 70is streaming via the networks 81 and 82 to the requesting terminal 71,which reproduces the streaming video data on a real-time basis.

Each terminal 71 can carry out two types of playback. One is normalplayback of reproducing video data at a constant speed. The other ishigh-speed playback (N-fold speed playback) of reproducing video data ata speed higher than the constant speed. For normal playback, theterminal 71 issues a playback request, and the content server 70 sendsout video data at a speed (constant speed) corresponding to the constantplayback speed of the terminal 71 for normal playback. For high-speedplayback, the terminal 71 issues a speed-change request, and the contentserver 70 sends out video data at a speed (high speed) corresponding tothe playback speed of the terminal 71 for high-speed playback. To stopplayback, the terminal 71 issues a stop request, and the content server70 stops sending out the video data.

The speed managing section 77 manages the playback speeds of theterminals 71. That is, the speed managing section 77 includes a storagearea for storing the playback speeds of the terminals 71. For example,based on the speed-change request from any of the terminals 71, thespeed managing section 77 updates the stored playback speed, therebymanaging the current playback speeds of the terminals 71.

The video cache section 75 includes a cache area 75 a for caching thevideo data received by the video receiving section 74 from the contentserver 70.

The video receiving section 74 receives the requested video data fromthe video cache section 75 if the requested video data is cached, orotherwise from the content server 70.

That is, when the request receiving section 73 receives a playbackrequest, the video receiving section 74 determines whether the requestedvideo data has been cached in the video cache section 75. If therequested video data has not yet been cached in the video cache section75, the video receiving section 74 starts receiving the video data fromthe content server 70.

The video streaming section 76 streams the video data cached by thevideo cache section 75 to the terminal 71 at a speed corresponding toeach playback speed.

The video receiving section 74 receives video data from the contentserver 70 at a speed Vmax corresponding to a speed that is the highestof the playback speeds of the terminals 71 managed by the speed managingsection 77. This makes it possible to stream video data to each terminal71 at a speed corresponding to each playback speed.

Furthermore, when the request receiving section 73 receives aspeed-change request for changing the speed Vmax, at which the videoreceiving section 74 is receiving video data, to another speed V, thevideo receiving section 74 compares the speed V with the speed Vmax. IfV>Vmax, the video receiving section 74 changes the operation to retrievevideo data from the content server 70 at the speed V. If V<Vmax, on theother hand, the video receiving section 74 continues receiving videodata from the content server 70 at the speed Vmax.

That is, if any of the terminals 71 has changed the playback speed tothe speed V, the speed V is compared with the maximum speed Vmax beforechange. If V>Vmax, the speed of video reception from the content server70 is changed from Vmax to V. On the other hand, if V<Vmax, the speed ofvideo reception is maintained at Vmax. This makes it possible to streamvideo data at a speed corresponding to the playback speed of eachterminal 71 even with any speed change at any terminal 71.

Also, when the request receiving section 73 receives a stop requestwhile the video receiving section 74 is receiving video data from thecontent server 70 at the speed Vmax, the video receiving section 74changes the operation to retrieve video data from the content server 70at a speed V′max corresponding to a speed that is the highest of theplayback speeds, managed by the speed managing section 77, of theterminals 71 for except the terminal that issued the stop request. Thatis, the video receiving section 74 compares the speed V′max with apredetermined speed Vdef. If V′max>Vdef, the video receiving section 74changes the speed of video reception from the content server 70 fromVmax to V′max.

In other words, if one of the terminals 71 that operates at the highestspeed is stopped, the speed of video reception from the content server70 is changed to the speed V′ max corresponding to the speed of theterminal that operates at the second highest speed. If any terminal 71other than the one that operates at the highest speed is stopped, thespeed is maintained at the speed Vmax. Thus, it is possible to streamvideo data at a speed corresponding to the playback speed of each of theremaining terminals 71 irrespective of which terminal is stopped.

As such, the playback speeds of the terminals 71 are managed, and videodata is received from the content server 70 at the speed correspondingto the highest speed of the playback speeds of the terminals 71.Therefore, video data can be streaming at the speed corresponding to theplayback speed of each terminal 71.

When the speed V′max is lower than the predetermined speed Vdef,however, the video receiving section 74 receives video data from thecontent server 70 not at the speed V′max but at the speed Vdef. That is,if V′max<Vdef, the video receiving section 74 changes the speed of videoreception from the content server 70 from Vmax to Vdef. Thus, even ifthe playback speed of any of the terminals 71 becomes extremely low, thespeed of video reception from the content server 70 is maintained at thespeed Vdef or higher for caching. Therefore, it is possible to respondto a sudden seek at any of the terminals 71.

An example hardware structure of the video proxy server according to thepresent embodiment is similar to that according to the first embodiment,which is illustrated in FIG. 2. The operations of the componentsaccording to the present embodiment are also similar to those accordingto the first embodiment. However, the program stored in the ROM 34 isdifferent from that according to the first embodiment, thereby achievingthe above-described operations of the components (73 to 77) of the videostreaming system of the second embodiment. Note that the program may beprovided as being stored in a portable recording medium such as CD-ROM,or may be supplied via a network.

Described below is the operation which is carried out by theabove-structured video proxy server 72 for changing the playback speed.FIG. 10 is a flowchart showing the operation which is carried out by thevideo proxy server 72 according to the present embodiment for changingthe playback speed. As shown in FIG. 10, in the video proxy server 72,the request receiving section 73 first determines whether a request hascome from any of the terminals 71 (step S21), and receives the request,if any (step S22). Then, the request receiving section 73 determineswhether the request is a playback request (step S23). If the request isnot a playback request, the procedure goes to step S25.

If the request is a playback request, the video proxy server 72 startsreceiving video data from the content server 70, caching the video data,and streaming the video data to the terminal 71 (step S24).

In the next step S25, it is determined whether the request received instep S22 is a stop request or a speed-change request. If the receivedrequest is neither a stop request nor a speed-change request, theprocedure returns to step S21, where the request receiving section 73waits until another request comes from any of the terminal 71.

If the received request is a stop request or a speed-change request, thereceiving speed is determined based on the playback speeds of theterminals 71. At the determined receiving speed, video data is receivedfrom the content server 70, temporarily cached, and streamed to eachterminal 71 at the speed corresponding to each playback speed (stepS26). The procedure then returns to step S21, where the requestreceiving section 73 waits until another request comes from any of theterminal 71.

The operation which is carried out by the video proxy server 72 forchanging the speed has now been described. Described next are detailsabout the aforementioned steps S24 and S26.

[Step S24]

The operation in step S24 of FIG. 10 which is carried out by the videoproxy server 22 for receiving video data from the content server 70,caching the video data, and streaming the video data to the requestingterminal 71 is similar to step S14 of FIG. 3 in the first embodiment,and its details are shown in FIG. 4. However, video data to be streamedto the terminal 71 is hereinafter denoted as “M0”, and a speed at whichthe video receiving section 74 receives the video data M0 from thecontent server 70 in step S102 of FIG. 4 is hereinafter denoted as “V0”,which is the maximum speed Vmax before speed change.

[Step S26]

FIGS. 11 to 13 show flowcharts showing details of operations which arecarried out by the video proxy server 72 for determining the receivingspeed based on the playback speeds of the terminals 71, receiving videodata at the determined speed, temporarily caching the video data, andstreaming the video data to each terminal 71 at the speed correspondingto each playback speed. With reference to FIG. 11, the video cachesection 75 is now caching the video data at a speed V0, and the videostreaming section 76 is now streaming the video data at the speed V0(step S401). In this state, any one of the terminals 71 stops playbackor changes a playback speed, and the request receiving section 73receives a request for stopping video streaming (stop request) or arequest for changing a video streaming speed (speed-change request)(step S402).

Then, it is determined whether the request received in step S402 by therequest receiving section 73 is a stop request (step S403). If thereceived request is a stop request, an operation denoted as “A” isexecuted, which is illustrated in FIG. 12.

In FIG. 12, when the request receiving section 73 receives a stoprequest, the video streaming section 76 stops streaming of the videodata M0 to the terminal 71 that made the stop request (step S501). Thevideo streaming section 76 then inquires the speed managing section 77of the playback speeds of the terminals 71 to which the video data M0 isstreaming. That is, assuming that the video data M0 is currentlystreaming to n terminals 71 (n is an arbitrary integer not less than 1and not more than (k−1)), streaming speeds V1 to Vn of these terminals71 are received (step S502). Then, the highest speed of the speeds V1 toVn (the speed is hereinafter denoted as MAX(V1, V2, Vn)) is comparedwith the predetermined speed Vdef (step S503).

If the speed Vdef is higher in step S503, the content server 70 isrequested to change the streaming speed of the video data M0 to MAX(V1,V2, . . . , Vn) (step S505). The procedure then returns to the flow ofFIG. 10.

Referring back to FIG. 11, if it is determined in step S403 that thereceived request is not a stop request, the request from the terminal 71is a speed-change request, and it is determined whether a requestedspeed V is higher than the original speed V0 (step S404). If therequested speed V is higher than the original speed V0, an operationdenoted as “B” is executed, which is shown in FIG. 13.

In FIG. 13, the video receiving section 74 requests the content server70 to change the streaming speed of the video data M0 to the speed Vrequested by the terminal 71 (step S601). The video streaming section 76changes the streaming speed of the video data MO to the terminal 71 thatmade the speed-change request to the requested speed V (step S602). Theprocedure then returns to the flow of FIG. 10.

Referring back to FIG. 11, if the speed V requested by the terminal 71is lower than the streaming speed V0 of the video data MO from thecontent server 70, the video streaming section 76 inquires the speedmanaging section 77 of the playback speeds of the terminals 71 to whichthe video data MO is streaming. That is, the streaming speeds V1 to Vnto n other terminals to which the video data MO is streaming arereceived from the speed managing section 77 (step S405). Then themaximum speed MAX(V, V1, V2, . . . , Vn) of the speeds V and V1 to Vn iscompared with the predetermined speed Vdef (step S406).

If the speed Vdef is higher in step S406, the content server 70 isrequested to change the streaming speed of the video data MO to thespeed Vdef (step S407). The procedure then goes to step S409.

On the other hand, if MAX (V, V1, V2, . . . , Vn) is higher than Vdef,the content server 70 is requested to change the streaming speed of thevideo data MO to MAX (V, V1, V2, . . . , Vn) (step S408) The procedurethen goes to step S409.

In the next step S409, the video streaming section 76 is instructed tochange the streaming speed of the video data MO to the terminal thatissued the speed-change request to the requested speed V. The procedurethen returns to the flow of FIG. 10.

As such, according to the present embodiment, the playback speeds of theterminals 71 are managed so that video data is received from the contentserver 70, cached, and streamed to the terminals 71 at a speedcorresponding to a speed that is the highest of all playback speeds ofthe terminals 71. Therefore, video data can be streaming at the speedcorresponding to the playback speed of each terminal 71.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A video proxy server, which is provided between a content serverstoring video data and a terminal capable of video playback, forreceiving the video data from the content server, caching the videodata, and streaming the video data to the terminal for playback, saidvideo proxy server comprising: a request receiving section for receivinga request from the terminal; a video receiving section for receiving thevideo data from the content server based on the request from theterminal; a video cache section for caching the video data received bysaid video receiving section; and a video streaming section forstreaming the video data cached by said video cache section to theterminal, wherein: when said request receiving section receives aplayback request, said video receiving section is operable to start avideo receiving operation of receiving the video data from the contentserver; when said request receiving section receives a seek playbackrequest while said video receiving section is executing the videoreceiving operation, said video receiving section is operable to startreceiving video data after a seek position from the content server whilecontinuing the video receiving operation; said video cache sectionincludes a cache area for caching the video data received by said videoreceiving section from the content server, and a temporary cache areafor temporarily caching the video data after the seek position receivedby said video receiving section from the content server; and when saidrequest receiving section receives the seek playback request, said videocache section is operable to cache in said temporary cache area thevideo data after the seek position while an operation of caching thevideo data in said cache area is not interrupted.
 2. The video proxyserver according to claim 1, wherein: when said request receivingsection receives the playback request, said video streaming section isoperable to start reading the video data from said cache area andstreaming the video data to the terminal; and when said requestreceiving section receives the seek playback request while said videoreceiving section is receiving the video data from the content server,said video streaming section is operable to stop reading the video datafrom said cache area and streaming the read video data to the terminal,and start reading the video data after the seek position cached in saidtemporary cache area and streaming the read video data after the seekposition to the terminal.
 3. The video proxy server according to claim1, wherein when said request receiving section receives a stop requestwhile said video receiving section is receiving the video data after theseek position from the content server in response to the seek playbackrequest, said video cache section is operable to clear said temporarycache area.
 4. The video proxy server according to claim 1, wherein whensaid request receiving section receives a next seek playback requestwhile the video receiving section is receiving the video data after theseek position from the content server in response to the seek playbackrequest, said video cache section is operable to clear said temporarycache area, and then start caching the video data after a next seekposition in said temporary cache area.
 5. The video proxy serveraccording to claim 1, wherein said video receiving section is operableto receive the video data or the video data after the seek positionbased on the request from the terminal only when the requested videodata or the requested video data after the seek position has not beencached by said video cache section.
 6. A video streaming method, whichis carried out between a content server storing video data and aterminal capable of video playback, for receiving the video data fromthe content server, caching the video data, and streaming the video datato the terminal for playback, said method comprising: receiving arequest from the terminal; receiving the video data from the contentserver based on the request from the terminal received in said receivingof the request; caching the video data received in said receiving of thevideo data; and streaming the video data cached in said caching of thevideo data to the terminal, wherein: in said receiving of the videodata, when a playback request is received in said receiving of therequest, a video receiving operation of receiving the video data fromthe content server is started, and when a seek playback request isreceived in said receiving of the request while the video receivingoperation is being executed, an operation of receiving video data aftera seek position from the content server is started while the videoreceiving operation is being continued; and in said caching of the videodata, when the playback request is received in said receiving of therequest, a video caching operation of caching the video data received insaid receiving of the video data from the content server is started, andwhen the seek playback request is received in said receiving of therequest while the operation of receiving the video data from the contentserver is being executed, the video caching operation being in executionis not interrupted and an operation of temporarily caching the videodata after the seek position received in said receiving of the videodata from the content server is started.
 7. A program recorded on acomputer-readable storage medium causing a computer to performoperations of a video streaming method, which is carried out between acontent server storing video data and a terminal capable of videoplayback, for receiving the video data from the content server, cachingthe video data, and streaming the video data to the terminal forplayback, said program causing the computer to perform operationscomprising: receiving a request from the terminal; receiving the videodata from the content server based on the request from the terminalreceived in said receiving of the request; caching the video datareceived in said receiving of the video data; and streaming the videodata cached in said caching of the video data to the terminal; whereinin said receiving of the video data, when a playback request is receivedin said receiving of the request, a video receiving operation ofreceiving the video data from the content server is started, and when aseek playback request is received in said receiving of the request whilethe video receiving operation is being executed, an operation ofreceiving video data after a seek position from the content server isstarted while the video receiving operation is being continued; andwherein in said caching of the video data, when the playback request isreceived in said receiving of the request, a video caching operation ofcaching the video data received in said receiving of the video data fromthe content server is started, and when the seek playback request isreceived in said receiving of the request while the operation ofreceiving the video data from the content server is being executed, thevideo caching operation being in execution is not interrupted and anoperation of temporarily caching the video data after the seek positionreceived in said receiving of the video data from the content server isstarted.